Accurate Current Mirror Circuit for Laser Drivers
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Solution Overview
Problem
Conventional laser drivers face challenges in achieving accurate modulation current due to insufficient voltage headroom, particularly in common-anode driving mode, where the offset voltage of error amplifiers affects current control accuracy, especially at low voltages.
Innovation Solution
An accurate current mirror circuit is introduced, comprising a reference current detection unit, a tail current source unit, and a control unit, which generates a bias voltage and reference voltage to regulate the mirror current, utilizing NMOS and NPN transistors to ensure stability and accuracy through feedback mechanisms, thereby mitigating the impact of offset voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional error amplifier with offset voltage is used in low voltage headroom, then circuit structure is simple, but current control accuracy deteriorates
Solution Approach 1:
The patent introduces an intermediary circuit (the current mirror circuit with feedback) between the error amplifier and the laser driver to eliminate the harmful effect of offset voltage. The feedback mechanism acts as a mediator that compensates for the offset voltage automatically, allowing the use of a simple error amplifier structure while achieving high current control accuracy.
Solution Approach 2:
The patent implements a feedback mechanism where the output current is monitored and fed back to adjust the input signal, thereby compensating for the offset voltage of the error amplifier. This feedback loop ensures that the actual current matches the desired current, resolving the contradiction between simple circuit structure and accurate current control.
2Measurement precision
If additional circuit is added to eliminate offset voltage, then current control accuracy is improved, but device complexity increases
Solution Approach 1:
The patent merges the offset voltage compensation function into the existing current mirror circuit structure, rather than adding a separate compensation circuit. By combining multiple functions (current mirroring, feedback, and offset compensation) into a unified circuit architecture, the patent achieves high current control accuracy without significantly increasing device complexity.
3Use of energy by moving object
If voltage headroom is reduced to meet supply voltage constraints, then power consumption is reduced, but current control accuracy deteriorates
Solution Approach 1:
The patent employs dynamic feedback adjustment that adapts to the available voltage headroom. The feedback mechanism automatically adjusts the control signal to compensate for the limited voltage range, enabling accurate current control even when the voltage headroom is reduced to meet power consumption constraints.
Data Source
AI summary
This invention provides an accurate current mirror circuit in a low voltage headroom applied to common-anode laser drivers, including a reference current detection unit, a tail current source unit, and a control unit. The reference current detection unit generates a bias voltage and a reference voltage according to a reference current from the reference current source; the tail current source unit receives the bias voltage and generate a mirror current accordingly; the control unit receives the reference voltage and an output voltage corresponding to the mirror current and carry out a feedback regulation to the bias voltage accordingly. In this invention, the reference voltage and the output voltage are locked at same level, and then the bias voltage is mirrored to generate the mirror current outputted to the laser, thus avoiding the problem of inaccurate current output caused by the offset of the control unit in the low voltage headroom.